Soft-Synchronization of Generators Using Controllable Inductive Fault Current Limiters

We describe a method for soft-synchronization of generators by controlling an inductive fault current limiter located between the generator and the grid. Our method limits the peak shaft torque and the frequency oscillations that normally occur following mistimed closure of the bus tie switch. The method improves the success rate of generator close-in, especially during emergencies or when operating with low transient-stability margin, with low inertia and, therefore, it lengthens the life of turbogenerators. This method introduces no adverse effects during an ideal (correctly timed) generator-synchronization process. We prove the success of the technique by using transient time-domain analysis and describe the control and an approach to analyze the soft-synchronization process that can be generally applied to either an infinite power system or to a limited-capacity system. After the generator is synchronized to a grid and operates stably, the transient-stability margin between the generator and the grid is not affected by this method. This soft-synchronization method has been validated by simulations with the increased critical margin of the rotor-angle difference and the improved faulty synchronization process with limited power impulse and frequency oscillation.